Brief preconditioning ischemia produces tolerance to subsequent prolonged ischemia that would otherwise cause brain injury. The genomic signature of the tolerant brain is transcriptional suppression. The development of tolerance, however, requires new protein synthesis, indicating that changes in protein expression contribute significantly to the mechanism of tolerance. To understand how, we characterized the proteome of the tolerant brain (Stapels et al. Sci Signaling, 2010). It is enriched in histone proteins and, remarkably, in polycomb group (PcG) proteins, which function as transcriptional suppressors. Thus, we may have discovered the mechanism that induces transcriptional suppression in tolerance. Our results implicate epigenetic regulation mediated by PcG proteins. Further, our results show that PcG proteins, previously known as regulators of segmentation during development in Drosophila, have a novel neuroprotective function in the brain. Our preliminary data on ischemic tolerance in vivo and in vitro show that the development of ischemic tolerance is dependent upon the expression of PcG proteins: knockdown ablates tolerance, and over- expression produces tolerance. Accordingly, we offer the following aims to establish and define PcG proteins role as actuators of tolerance. Aim 1. To identify early, differential changes in PcG protein abundance and activity during the induction of tolerance. We will characterize changes in the expression of PcG proteins within different polycomb protein repressive complexes (PRCs). We will also characterize PcG protein- mediated histone modifications during the development of tolerance modeled in mice, over time. The results will define which PcG proteins and complexes participate in tolerance; demonstrate a rapid increase in PcG protein abundance at the initiation of tolerance; and establish epigenetic regulation through histone modi- fication as a mechanism underlying ischemic tolerance. Aim 2. To establish an essential role for PcG proteins in the development of ischemic tolerance. We will evaluate the effect of PcG protein expression on the outcome of ischemia using loss-of-function and gain-of-function approaches. Knockdown or over'expression of PcG proteins will be achieved by using small hairpin RNA (shRNA) or recombinant cDNA, respectively, both in vivo and in vitro. The results will demonstrate that the abundance of PcG proteins profoundly affects the outcome of ischemia. Aim 3. To demonstrate that PcG proteins control the expression of genes that are suppressed in ischemic tolerance. We will investigate the interaction of PcG proteins with the promoters of genes downregulated in tolerance using ChIP assays. When genes encode channels, electrophysiological analyses of cultured neurons, over- or under-expressing PcG proteins, will be performed to establish the effect of PcG proteins on the activity of tolerance effectors. We will also manipulate PcG proteins and potassium channels simultaneously and examine the effect on tolerance induction in mice in vivo. The results will show that tolerance effector genes and gene products can be modulated by alterations in PcG protein abundance.